Summary The Robinson R22B helicopter, C-FHRL, serial number 1361, was operating with a flying instructor and a student on board. About 10 nautical miles east of the Abbotsford airport, the helicopter was seen to break apart in flight and fall to the ground. No one observed the helicopter before the accident sequence began; however, several persons saw it descend in a flat attitude and pieces fall from it. They also observed that the main rotor was stationary and that the blades were coned. A mist with a strong smell of fuel was reported by the first person to arrive at the scene. No fire occurred. Both occupants were fatally injured. Ce rapport est galement disponible en franais.. Other Factual Information The instructor was conducting his first instructional flight with a student since qualifying for a Class4 helicopter instructor rating. He held a commercial helicopter pilot licence and a type rating for the Robinson R22. He had flown about 2900hours on helicopters, of which approximately 1600hours were on the RobinsonR22 and R44. His last medical, which included an electrocardiogram, was conducted on 18April2001 and was assessed as Category1 with no limitations. The student was receiving his initial familiarization flight as a student helicopter pilot. His medical examination was conducted on 25April2001, and he was assessed as Category1, with the limitation that glasses must be worn. Autopsies of the instructor and the student, including a full toxicology screening, did not reveal any conditions that could have led or contributed to the accident. At the time of the accident, the weather conditions were suitable for flight in accordance with visual flight rules. The actual weather at the Abbotsford airport, 10nautical miles west-southwest of the accident site, at 1400 Pacific daylight time,(1) 37minutes before the accident, was as follows: wind 240 true at 12knots gusting to 18knots; visibility 25statute miles; a few clouds at 2500feet, broken clouds at 4500feet, and broken clouds at 21000feet; temperature 14 C; and dew point 5 C. Weather data recorded at an agricultural locale bordering the accident site showed the wind speed to be 6knots gusting to 14knots at the approximate time of the accident. The wind direction was not recorded. According to the carburettor icing chart in Aeronautical Information Publication, section AIR 2.3, a temperature of 14 C and a dew point of 5 C falls right at the boundary between serious icing at any power and moderate icing at cruise power or serious icing at descent power. The carburettor heat control was found in the OFF position. Logbooks and maintenance records indicate that the Robinson R22B had been certified, equipped, and maintained in accordance with existing regulations and approved procedures. It was reported that, on departure from Abbotsford, the helicopter had 50litres of fuel on board, which would give an endurance of about two hours. The accident occurred about 20minutes after the helicopter took off. The helicopter had no known deficiencies before the flight and was operating within its load and centre-of-gravity limits. The wreckage was initially examined at the accident site. The helicopter struck the ground in a slightly right-banked, flat attitude. The debris field indicates that the helicopter was either stopped or proceeding very slowly in a north-northeasterly direction. The main-rotor blades were found bent into a tulip shape. This indicates low rotor-rpm during the descent to the ground. The wreckage was recovered from the accident site for a more detailed examination at the TSB regional wreckage examination facility. Paint transfer marks indicate that the main-rotor blades contacted the fuselage at the strobe light mount area and chopped off the tail boom. This is supported by the deformation of both main-rotor blade spindle tusks, indicating excessive blade flap downward. The breaks on the tail-rotor drive shaft exhibited signature blade strikes to the tail boom at low rpm. The main-rotor hub assembly showed some indications of mast bumping. The mast was bent immediately below the hub, but these indications were not severe. (In most instances of high-energy rotor/fuselage contact, the indications are severe.) Both teeter and droop stops were found in place. The main-rotor hub displayed smile marks on both sides from contact with the main-rotor pitch horn circumferences. These marks are consistent with both blades being coned upwards. A teardown and an inspection of the Lycoming engine (modelO-320-B2C, serial numberL-7020-39A) was conducted at a Lycoming overhaul facility in Richmond, British Columbia, on 29May2001. Engine damage noted during this teardown was consistent with the engine contacting the ground and the airframe during the accident. The engine appeared to be mechanically capable of producing power before the accident, although it was not operating at impact. No indication was found of pre-existing damage or defects that could have contributed to the accident circumstances. The engine was fitted with an electronic fuel control governor to help the pilot maintain rotor rpm and reduce the risk of low rotor-rpm leading to rotor stall. This installation featured a governor on/off switch on the end of the student pilot's collective lever. This collective governor switch was destroyed in the crash. All governor components, both magnetos, and the governor off and low-rpm warning light bulbs were sent to the TSB Engineering Laboratory for analysis. It was not determined whether the governor was operating when the accident sequence began, but information from laboratory analysis suggests that the governor was off at impact. With two persons on board and full fuel, the R22 is operating at close to its maximum gross weight of 1370pounds. This results in operations being routinely conducted near the upper limit of this helicopter's operating envelope, which, in turn, is near the maximum design lift capability of the main-rotor system. To gain the needed lift, the R22's main-rotor blade angle of attack will on occasions be near the stall angle of attack during normal operations. A simulation study conducted by the Georgia Institute of Technology revealed that large, abrupt control movements may cause a rapid decay of the rotor rpm due to the low inertia of the main rotor. The most effective technique to recover from low rotor-rpm is to immediately lower the collective to decrease the blades' angle of attack, flare to transfer airspeed energy to rotor energy, and roll on throttle. In this accident, mast bumping marks and other damage to the main-rotor hub assembly are consistent with a low-energy situation, which would correspond to a low-rotor-rpm stall. Rotor stall due to low rpm has resulted in many helicopter accidents. At the stalling angle, usually around 15 , the airflow over the rotor blades will abruptly separate, causing a sudden loss of lift and a large increase in drag. A rotor stall occurs because of low rotor-rpm. As the rotor rpm decreases, the angle of attack of the rotor blades must be increased to generate the lift required to support the helicopter, else the helicopter will descend. Once the rotor blades reach the stalling angle of attack, lift suddenly decreases and drag greatly increases. This increased drag acts like a huge rotor brake, causing the rotor rpm to decrease further, accentuating the effect of the rotor stall. Once the rotor rpm has decayed significantly, recovery is unlikely because, as the helicopter begins to descend, the upward rushing air further increases the angle of attack of the slowly rotating blades. A tail boom chop often accompanies a low-rotor-rpm stall because of asymmetrical rotor stall, that is, the tendency for the helicopter to pitch nose-down due to the upward airflow under the tail surfaces and the application of aft cyclic by the pilot in an attempt to keep the nose from dropping. A search of the TSB database has revealed that in Canada since 1993 there have been nine similar R22 helicopter occurrences in which rotor rpm was allowed to decay. A search of the US National Transportation Safety Board database revealed 27similar occurrences in the US since 1983.